1. Field of the Invention
The invention relates to crosslinkable compositions of silane-crosslinking prepolymers and to their methods of making and use as adhesive and sealant materials, in particular for adhesive bonding of substrates.
2. Description of the Related Art
Polymer systems having reactive alkoxysilyl groups are well known. On contact with water and/or atmospheric humidity, these alkoxysilane-terminated polymers are able to condense with each other even at room temperature by eliminating the alkoxy groups. One of the most important applications for such materials is in the manufacture of adhesives, particularly the manufacture of adhesive systems that are elastic.
This is because adhesives based on alkoxysilane-crosslinking polymers exhibit very good mechanical properties in the cured state, since they are able to combine tensile strength with high elasticity. A further decisive advantage of silane-crosslinking systems over numerous other adhesive and sealant technologies (over isocyanate-crosslinking systems, for example) is the fact that the prepolymers exhibit no concerns. There are many applications in this area where there is a preference for one-component systems (1K systems) that cure on contact with atmospheric humidity.
Nearly all silane-crosslinking adhesive and sealant materials contain what are known as water scavengers. These are usually indispensable for an economical manufacturing process, since it is generally not carried out under perfect protective-gas conditions, causing trace moisture to become entrapped in the product during formation and packaging. Moreover, the solid fillers included in what are known as “filled systems” always entrain a certain amount of moisture absorbed on the particle surfaces. Without water scavengers, these trace amounts of water would lead directly to a condensation reaction involving at least some of the polymer molecules and hence to some undesired stiffening of the resulting composition.
Moreover, most of the containers in which adhesive and sealant materials are stored are neither absolutely watertight nor made of materials that would completely prevent any diffusion of water. Again, water scavengers are indispensable here to ensure adequate stability in storage.
Water scavengers work by chemically reacting with the trace water entrained in the particular product, so the trace water is no longer available for a reaction with the silane-crosslinking polymers.
Vinyltrimethoxysilane is virtually always used as a water scavenger in silane-crosslinking systems. EP 0 918 062 A1 may be cited here as representative of the many documents in which this practice is described, for its clear overview and description of typical ingredients in silane-crosslinking adhesive and sealant materials.
The reason for the indispensability of vinyltrimethoxysilane in nearly all commercially relevant silane-crosslinking adhesive and sealant formulations is particularly the high reactivity of this silane as well as its inexpensive availability: the half-life of vinyltrimethoxysilane in an aqueous solution is just 855 s at pH 7 and <10 s at pH 9. That is, more than half of all silane molecules will have reacted with water at the end of this period. Half-lives of silanes without Si-bonded vinyl group are distinctly longer here in that, for instance, methyltrimethoxysilane has a half life of 3360 s at pH 7 and 111 s at pH 9, 3-methacryloyloxypropyltrimethoxysilane has half-lives of 8500 s at pH 7 and 185 min at pH 9, and the half-lives of 3-glycidoxypropyltrimethoxysilane are 6500 s at pH 7 and 240 s at pH 9.
Since most silane-terminated organic polymers have silyl groups attached to the polymer moiety via a propyl spacer, their reactivities are comparable to those of the last two polymers mentioned, which are likewise propyl spaced. Vinyltrimethoxysilane accordingly has not just a higher reactivity toward water than virtually all other silanes, but also a higher reactivity than the silane-crosslinking polymer to be protected by it from any premature reaction with moisture. This is one very important property for a water scavenger.
Only the so-called alpha-silanes as described in EP 1414909 A for example are even more reactive than vinyltrimethoxysilane. They have a reactive alkoxysilyl group attached through a methylene spacer to an adjacent heteroatom. This arrangement, like the vinyl group of vinyltrimethoxysilane, leads to an appreciable increase in the reactivity of the silyl group in question. However, alpha-silanes are burdensome to synthesize and so are significantly more costly than vinyltrimethoxysilane and therefore do not represent an economically sensible alternative.
However, notwithstanding its extremely wide use, vinyltrimethoxysilane also has some disadvantages. Having a boiling point of 122° C. at 1013 mbar, this silane is relatively volatile and also has an odor which some users perceive as very unpleasant. Similarly, its ISO 13736 flashpoint is, at just 25° C., relatively low, compromising its processability. Yet these disadvantages notwithstanding, to date it is the advantages such as the low cost and also the good mechanical properties of the corresponding compositions after curing which have hitherto made vinyltrimethoxysilane an indispensable ingredient in almost all common formulations of adhesive and sealant materials.